1. Field of the Invention
The present invention relates generally to the field of apparatus for fiber optic communication, and more particularly, to a multi-channel fiber optic rotary joint using de-rotating mechanism in telecommunication industry.
2. Description of Related Art
A typical fiber optical rotary joint consists of a fixed fiber collimator holder and a rotatable fiber collimator holder which are relatively rotatable each other to allow uninterrupted transmission of optical signals through the rotational interface from fiber collimators on any one of the holders to the fiber collimators on another holder.
The multi-channel fiber optic rotary joints of prior arts typically utilize an opto-mechanical de-rotating mechanism between the fixed fiber collimator holder and the rotatable fiber collimator holder. An opto-mechanical de-rotating mechanism includes an optic de-rotating element, and a mechanical speed change unite. The optic de-rotating element can be Dove prism, Delta prism, Abbe-Konig prism, Schmidt-Pechan prism, and a cylindrical GRIN lens. It rotates at half the speed of rotation of the rotatable fiber collimator holder. The mechanical speed change unite, usually a gear system, provides 2:1 speed ratio for the rotatable fiber collimator holder and the optic de-rotating element.
In a gear system, when two gears mesh each other, there is usually a clearance or backlash between the teeth of two gears due to manufacturing errors and assembly errors. At a reversal transmission, the direction of rotation changes and the output shaft of gear transmission system would turn a slight angle due to the above named clearance, or backlash. That would cause a motion loss, or kinematic transmission error and dynamically also cause noise and vibration. So the anti-backlash mechanism is usually used in gear system.
The examples of the prior arts include U.S. Pat. No. 5,157,745, U.S. Pat. No. 5,371,814, U.S. Pat. No. 5,442,721, and U.S. Pat. No. 6,301,405, where a Dove prism is commonly used as optic de-rotating element.
Variety of gear systems are designed to provides 2:1 speed ratio. Usually one gear is fixed with rotor; another gear fixed with Prism holder, or prism rotor; while other gears engage with the respective gears to transform motion from rotor to prism holder. Like U.S. Pat. No. 5,371,814, a planetary gear subassembly having an internal gear mounted for rotation with the image-derotation-prism, a star gear mounted for rotation with the second lens housing member, and a plurality of planetary gears journalized to the stator between and mechanically ganged with said star gear and said internal ring gear. In U.S. Pat. No. 5,442,721, a two to one gear assembly consists of 4 external gears. Two of them are fixed with rotor and prism holder respectively. The left two gears mounted on a common shaft to connect said gear on rotor and said gear on prism holder. Similarly in U.S. Pat. No. 5,157,745, an internal gear mounted to said first rotor; an external gear mounted to said second, Dove prism holding, rotor; and two drive gears mounted on an axle for connecting said internal gear to said external gear. The anti-backlash mechanism consists of two springs to adjust the position of said axle to allow the drive gears to be held tightly against the respective internal gear and external gear by spring force.
Another kind of anti-backlash mechanism can be seen in U.S. Pat. No. 6,301,405, where more gears and springs are utilized to enable tight engagement of each gear set.
There are at least two common disadvantages in the prior arts: first, many more external gears are used in the gear system than internal gear; second, wire spring, e.g., compression spring or extension spring, are used in the anti-backlash mechanism. Both of them require more space for designed Fiber optic rotary joint, which results in diameter increase of designed Fiber optic rotary joint.
Comparing with external gear, internal gear has many advantages. First, it allows a compact parallel shaft transmission drive. Used with a spur pinion the ratio is the same as that of two external gears, but the center distance is much smaller. Second advantage of internal gear is to reduce sliding action due to a convex profile surface working against a concave surface and as friction results in tooth wear; a reduction in the amount of sliding action is desirable. Another advantage is that internal gear usually provides a higher contact ratio to assure smooth continuous motion transmission. Internal gears also have better load-carrying capacity than external spur gears.
Flexure mechanisms or compliant mechanisms are flexible mechanisms that rely on elastic body deformation to produce motion and transmit forces. These are usually monolithic (single-piece) or jointless structures with certain advantages over the rigid-body, or jointed, mechanisms. The key advantage of flexure mechanisms lies in their simplicity and high precision in the absence of friction and backlash. Since the compliant mechanisms are single-piece structures, there is no need for assembly. With no joints, “rubbing” between two parts or friction as seen at the joints of rigid body mechanisms is absent. They cost less to make than the jointed variety.